Improved catalytic hydrocarbon conversion processes have created interest in utilizing olefinic feedstocks, such as petroleum refinery streams rich in lower olefins, for producing C.sub.5.sup.+ gasoline, diesel fuel, etc. In addition to the basic work derived from ZSM-5 type zeolite catalyst research, a number of discoveries have contributed to the development of a new industrial process, known as Mobil Olefins to Gasoline/Distillate ("MOGD"). This process has significance as a safe, environmentally acceptable technique for utilizing refinery streams that contain lower olefins, especially C.sub.2 -C.sub.5 alkenes. This process may supplant conventional alkylation units. In U.S. Pat. Nos. 3,960,978 and 4,021,502, Plank, Rosinski and Givens disclose conversion of C.sub.2 -C.sub.5 olefins, alone or in admixture with paraffinic components, into higher hydrocarbons over crystalline zeolites having controlled acidity. Garwood et al have also contributed improved processing techniques to the MOGD system, as in U.S. Pat. Nos. 4,150,062, 4,211,640 and 4,227,992. The above-identified disclosures are incorporated herein by reference.
Conversion of lower olefins, especially propene and butenes, over H-ZSM-5 is effective at moderately elevated temperatures and pressures. The conversion products are sought as liquid fuels, especially the C.sub.5.sup.+ aliphatic and aromatic hydrocarbons. Olefinic gasoline is produced in good yield by the MOGD process and may be recovered as a product or recycled to the reactor system for further conversion to distillate-range products.
As a consequence of the relatively low reactivity of ethylene with known zeolite oligomerization catalysts (about 10-30% conversion for H-ZSM-5), distillate-mode reactor systems designed to completely convert a large ethylene component of feedstock would require much larger size than comparable reactor systems for converting other lower olefins. Recycle of a major amount of ethylene from the reactor effluent would result in significant increases in equipment size. By contrast, propene and butene are converted efficiently, 75 to 95% or more in a single pass, under catalytic conditions of high pressure and moderate pressure used in distillate mode operation. In U.S. Pat. No. 4,433,185, Tabak employs a two stage conversion process, with interstage flashing of unconverted ethene and subsequent high severity ethene conversion.
Ethylene has substantial value as a feedstock for polymer manufacture or other industrial processes, and can be recovered economically. It has been found that an olefin-to-distillate process utilizing C.sub.2 -C.sub.4 olefinic feedstock can be operated to prefractionate the feedstock for ethylene recovery and catalytic conversion of the C.sub.3.sup.+ olefinic components.
Olefinic feedstocks may be obtained from various sources, including fossil fuel processing streams, such as gas separation units, cracking of C.sub.2.sup.+ hydrocarbons, coal byproducts, alcohol conversion, and various synthetic fuel processing streams. Cracking of ethane and conversion of effluent is disclosed in U.S. Pat. No. 4,100,218 and conversion of ethane to aromatics over Ga-ZSM-5 is disclosed in U.S. Pat. No. 4,350,835. Olefinic effluent from fluidized catalytic cracking of gas oil or the like is a valuable source of olefins, mainly C.sub.3 -C.sub.4 olefins, suitable for exothermic conversion according to the present MOGD process. It is an object of the present invention to provide a unique prefractionation system for recovery of valuable ethylene and economic operation of an integrated MOGD type reactor system.